Diesel engines have long been used to provide power generation in variety of applications, including industrial and vehicular, while achieving higher thermal efficiency than the popular spark-ignited engine. The higher efficiency of diesel engines results from the ability to create higher compression ratios while controlling the diesel engine's power output without a throttle. This eliminates the throttling losses of spark-ignited engines and results in significantly higher efficiency at part load for diesel engines.
Many diesel engines include a turbocharger for increasing the power output of the engine by increasing the mass flow rate of air to each cylinder thereby allowing a corresponding increase in the fuel flow rate. However, engine transient response of turbocharged diesel engines is limited by the turbocharger's inability to go from turbine wheel idle or low load operating speeds to full load operating speeds in an expedient manner. In fact, the fuel flow must be gradually increased to fall load conditions to match the response of the turbocharger so as to not create smoke due to a low air fuel ratio. One way to improve transient response is to utilize smaller turbochargers with lower inertias. However, smaller turbochargers undesirably limit peak engine performance. Also, regardless of the size of the turbocharger, diesel engines may produce an undesirably high level of unburned hydrocarbons (UHC) at reduced idle. Moreover, turbocharged diesel engines are sometimes unable to provide sufficient torque at low engine speeds as required in certain applications.
U.S. Pat. No. 5,233,948 to Boggs et al. discloses a variable cycle engine which varies the timing of an exhaust valve event at low load engine conditions to achieve high engine efficiency. During idle conditions, exhaust valve opening is maintained at less than 90 degrees before bottom dead center (BDC). As the load increases to a light load condition, the system retards exhaust valve opening by 35 degrees. As a result, the exhaust valves are not advanced from the conventional timing. Moreover, this reference no where suggest using the valve timing system with a turbocharged diesel engine. In addition, the system necessarily changes the timing of both exhaust valve opening and closing.
U.S. Pat. No. 4,469,056 to Tourtelot, Jr. discloses a variable valve timing system including a valve operating device including a rocker lever activated by a cam having a profile which opens an exhaust valve earlier in the expansion stroke of an engine piston to optimize engine efficiency at predetermined operating conditions. However, the exhaust valve opening is advanced an insufficient number of degrees before BDC. Also, the system disadvantageously retards exhaust valve closing when causing an advance in exhaust valve opening.
Consequently, there is a need for an improved variable valve timing control system for selectively causing an early opening of an exhaust valve so as to improve turbocharged diesel engine performance.